Air forces are very choosy about their pilot candidates (even in 1940) and don't mind washing out lots of them.
Indeed - my grandfather was rejected twice by the RAF in 1939/40, despite having a pilots licence and a fair few hours thanks to the Oxford UAS. He first applied to be a pilot and then a second time to be a navigator and was rejected on health grounds both times (he then joined the Army who were perfectly happy to have him).